Hydraulic apparatus with return to neutral mechanism

ABSTRACT

A control mechanism for a hydraulic drive apparatus such as a transaxle having a hydraulic pump and moveable swash plate mounted in a housing including a control arm for moving the swash plate between forward and reverse positions and a unidirectional return to neutral mechanism for biasing and returning the swash plate to a neutral position when the control arm is in one of the reverse or forward positions but not from the other position.

RELATED APPLICATION

This application is a continuation-in-part of and claims the benefit ofU.S. application Ser. No. 09/789,419, filed on Feb. 20, 2001 now U.S.Pat. No. 6,487,857, which is incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

This invention relates generally to axle driving apparatus and, moreparticularly, to a return to neutral mechanism adapted to be mounted tothe casing of a transaxle such as zero-turn transaxle. The return toneutral mechanism disclosed herein could also be used with hydraulicpumps or other types of hydrostatic transaxies.

Transaxle and hydrostatic transmission assemblies (“HSTs”) are known inthe art. Generally, an HST includes a center section on which is mountedto a rotating hydraulic pump and a rotating hydraulic motor. Thehydraulic pump and the hydraulic motor each carry a plurality ofreciprocating pistons which are in fluid communication through hydraulicporting formed in the center section. Rotation of the hydraulic pumpagainst a moveable swash plate creates an axial motion of the pumppistons that forces an operating oil through the hydraulic porting tothe hydraulic motor to move the motor pistons. The axial motion of themotor pistons causes the hydraulic motor to rotate as the motor pistonsbear against a thrust bearing. In this manner, the rotation of thehydraulic motor may be used to drive the vehicle axles of a riding lawnmower, small tractor and the like.

To adjust the speed and direction of rotation of the hydraulic motorand, accordingly, the speed and direction of rotation of the vehicleaxles, the position of the swash plate with respect to the hydraulicpump pistons may be changed. The orientation with which the swash plateaddresses the hydraulic pump pistons can be changed to control whetherthe hydraulic motor rotates in the forward direction or in the reversedirection. Additionally, the angle at which the swash plate addressesthe hydraulic pump pistons can be changed to increase or decrease theamount of operating oil that is forced from the hydraulic pump to thehydraulic motor to change the speed at which the hydraulic motorrotates.

For use in changing the position of the moveable swash plate, it isknown to include a trunnion arm that is coupled to the swash plate. Aspeed change lever or a speed change pedal is, in turn, coupled to thetrunnion arm through a wire or other driving link. In this manner,movement of the speed change lever/pedal results in movement of thetrunnion arm to change the position of the swash plate to therebycontrol the speed and direction of the vehicle. Examples of suchmechanisms for adjusting the speed of a vehicle may be seen in U.S. Pat.Nos. 6,122,996 and 5,819,537 which are incorporated herein by referencein their entirety.

For placing the swash plate in a position that neither effects the speednor the direction of rotation of the hydraulic motor, i.e., the neutralposition, known hydraulic pumps and hydrostatic transaxles provide areturn to neutral mechanism that is normally implemented as an integralpart of the vehicle linkage. While these return to neutral mechanismswork for their intended purpose, they do suffer disadvantages. Forexample, these known return to neutral mechanisms fail to allow forflexibility whereby different types and orientations of driving linkagesmay be used in connection with the hydraulic pumps and hydraulictransaxles.

SUMMARY OF THE INVENTION

To overcome these disadvantages, the present invention is realized in animproved speed adjusting mechanism having an integral return to neutralmechanism that is adapted to be mounted to the casing of a hydraulicpump or hydrostatic transaxle. For simplicity of explanation theinvention will be described in association with an integrated zero turntransaxle (IZT). The IZT includes a hydraulic transmission mountedwithin the casing that includes a rotatable hydraulic pump in fluidcommunication with a rotatable hydraulic motor and a moveable swashplate cooperable with the rotatable hydraulic pump for controlling thespeed and direction of rotation of the hydraulic motor. The rotation ofthe hydraulic motor is used to drive a single axle shaft.

For controlling the positioning of the swash plate, the transaxlefurther includes a rotatable trunnion arm coupled to the moveable swashplate. The rotatable trunnion arm extends from the casing and is coupledto the speed adjusting mechanism. The speed adjusting mechanism ismounted to the casing and is used to rotate the trunnion arm to changethe orientation of the swash plate to change the speed and direction ofrotation of the hydraulic motor.

More specifically, the speed adjusting mechanism includes a return armadapted to be mounted to the casing in a fixed position indicative of aneutral position of the trunnion arm. The neutral position of thetrunnion arm is the position of the trunnion arm in which the swashplate does not influence the speed and direction of rotation of thehydraulic motor. Additionally, the speed adjusting mechanism includes acontrol arm that is mounted to and moves the trunnion arm. A pair ofscissor return arms are provided that are adapted to move the controlarm in cooperation with the return arm for the purpose of moving thetrunnion arm to the neutral position. To provide additional mountingflexibility, the control arm is adapted to be mounted to the trunnionarm in any one of a plurality of different positions and the return armis capable of being mounted to the casing in a corresponding positionsuch that the return arm can be aligned with the control arm toestablish the neutral position.

In a further embodiment of this invention, the return to neutral featureis unidirectional, in that it provides a return force when the unit isstroked in one direction, either reverse or forward, but does notprovide any return force when the unit is stroked in the oppositedirection. In certain applications, the user may want to have such areturn force only when the unit is in reverse, for operational purposes,but not want to have it in the forward direction, since the maintenanceof the force needed to overcome the return force may be tiring to theuser, or may be otherwise unnecessary.

A better understanding of the objects, advantages, features, propertiesand relationships of the invention will be obtained from the followingdetailed description and accompanying drawings which set forth anillustrative embodiment and which are indicative of the various ways inwhich the principles of the invention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a pair of integrated, zero-turn transaxles(“IZTs”) each having a speed adjusting mechanism constructed inaccordance with the principles of the subject invention;

FIG. 2 is a front plan view of the IZTs of FIG. 1;

FIG. 3 is an exploded view of a further speed adjusting mechanism foruse in connection with the IZTs of FIGS. 1 and 2;

FIG. 4 is a cross-sectional view of the speed adjusting mechanism ofFIG. 3;

FIG. 5 is a front plan view of the embodiment shown in FIG. 3, with thespeed adjusting mechanism in the neutral position;

FIG. 6 is a front plan view of the embodiment shown in FIG. 3, with thespeed adjusting mechanism in the forward throttle position;

FIG. 7 is a front plan view of the embodiment shown in FIG. 3, with thespeed adjusting mechanism in the reverse throttle position; and

FIGS. 8a-8 h are exemplary orientations of a speed adjusting mechanismwhen mounted on the IZTs of FIGS. 1 and 2.

FIG. 9 is a front elevational view of an alternative embodiment of thisinvention incorporating a unidirectional return to neutral mechanism.

FIG. 10 is an exploded view of the embodiment shown in FIG. 9.

FIG. 11 is a front elevational view of the embodiment shown in FIG. 9,with the speed adjusting mechanism in the reverse direction.

FIG. 12 is a front elevational view of the embodiment shown in FIG. 9,with the speed adjusting mechanism in the forward direction.

FIG. 13 is a front elevational view of a further alternative embodimentof a unidirectional return to neutral mechanism of this invention.

FIG. 14 is a top plan view of the embodiment shown in FIG. 13.

FIG. 15 is a front elevational view of the embodiment shown in FIG. 13,with the speed adjusting mechanism in the reverse direction.

FIG. 16 is a front elevational view of the embodiment shown in FIG. 13,with the speed adjusting mechanism in the forward direction.

DETAILED DESCRIPTION OF THE DRAWINGS

Turning now to the figures, wherein like reference numeral refer to likeelements, there is illustrated an integrated, zero-turn transaxle 10(“IZT”). As described in greater detail in U.S. Pat. No. 6,152,247,which is incorporated herein by reference in its entirety, theillustrated IZT operates on the principle of an input shaft driving ahydraulic pump which, through the action of its pistons, pushes oil to ahydraulic motor through a center section to cause the rotation of amotor shaft. The rotation of the motor shaft is eventually transferredthrough a gearing system or the like to drive a single axle shaft 12. Asparticularly illustrated in FIGS. 1 and 2, the axle shaft 12 may bepositioned for either left-handed or right-handed drive.

For adjusting the amount of oil that is pushed from the hydraulic pumpto the hydraulic motor, the IZT 10 includes a moveable swash plateagainst which the pump pistons travel. As will be understood by those ofordinary skill in the art, the swash plate may be moved to a variety ofpositions to vary the stroke of the pump pistons and the direction ofrotation of the hydraulic motor. As the stroke of the pump pistons isvaried, the volume of the hydraulic fluid pumped into the hydraulicporting of the center section will vary. Since the speed of rotation ofthe hydraulic motor is dependent upon the amount of hydraulic fluidpumped thereinto by the hydraulic pump and the direction of rotation ofthe hydraulic motor is dependent upon the direction of rotation of thehydraulic pump, the positioning of the swash plate is seen to controlthe speed and direction of rotation of the hydraulic motor and,accordingly, the speed and direction of rotation of the axle shaft 12.

For moving the swash plate, the swash plate assembly is connected to amoveable trunnion arm 14 that is rotatably supported in the casing 11 ofthe IZT 10. As will be appreciated, rotation of the trunnion arm 14changes the angular orientation of the swash plate assembly with respectto the pump pistons. To rotate the trunnion arm 14 and, accordingly,move the swash plate assembly, a speed adjusting mechanism 16 is coupledto the trunnion arm 14. The speed adjusting mechanism 16 may beconnected, via a driving link, to a lever or a pedal provided on avehicle whereby movement of the lever or pedal is translated to thespeed adjusting mechanism 16 to cause the rotation of the trunnion arm14 and movement of the swash plate assembly. Since the trunnion arm 14extends from the IZT casing 11 to engage the speed adjusting mechanism16, a seal 30 can be placed around the trunnion arm 14 to preventleakage of hydraulic fluid from the opening in the IZT casing 11 fromwhich the trunnion arm 14 extends.

For use in rotating the trunnion arm 14, the speed adjusting mechanism16, illustrated more clearly in FIGS. 3-7, is comprised of a return arm20, a control arm 22, an inner scissor return arm 24, and an outerscissor return arm 26. A nut 34, which would be mated with correspondingthreads on the end of the trunnion arm 14, retains the return arm 20,control arm 22, inner scissor return arm 24, and outer scissor returnarm 26 on the trunnion arm 14. As will be described in greater detailbelow, the speed adjusting mechanism also functions to substantiallyestablish the neutral position of the trunnion arm 14, i.e., theposition of the trunnion arm 14 where the swash plate assembly does notinfluence the flow of hydraulic fluid within the hydraulic circuitformed between the hydraulic pump and the hydraulic motor, and to biasand move the trunnion arm 14 towards this neutral position.

To provide for rotation of the trunnion arm 14, the control arm 22 isnon-rotatably mounted to the end of the trunnion arm 14. Thenon-rotatable mating of the control arm 22 to the trunnion arm 14 ispreferably accomplished by providing the control arm 22 and trunnion arm14 with complimentary mating shapes. By way of example, the trunnion arm14 can be provided with a square shaped end that is adapted to mate witha corresponding square shaped opening in the control arm 22. In thismanner, rotation of the control arm 22 will also result in rotation ofthe trunnion arm 14. For moving the control arm 22 and, accordingly, thetrunnion arm 14 and the swash plate assembly to thereby control thespeed and direction of rotation of the axle shaft 12, the control arm 22includes openings 22 c to which hand/foot/electronically operateddriving links may be attached.

During assembly, the control arm 22 is mounted to the trunnion arm 14with the return arm 20 positioned between the control arm 22 and the IZTcasing 11. The inner scissor return arm 24 and the outer scissor returnarm 26 are mounted adjacent to the control arm 22 before the nut 34 ismated with the end of the trunnion arm 14. A biasing means 36, such as aspring, is linked to the inner and outer scissor return arms 24/26, inparticular, to arms 24 b/26 b of the inner and outer scissor return arms24/26 respectively. It will be appreciated that various types of biasingmeans are available, as exemplified by the two different types ofsprings illustrated in FIGS. 1 and 2 and in FIGS. 3-8. Once thecomponents are mounted to the trunnion arm 14, the control arm 22 ismoved until the trunnion arm 14 places the swash plate in the neutralposition.

When the trunnion arm 14 is placed in the position that corresponds tothe neutral position of the swash plate, the return arm 20 is attachedto the IZT casing 11. For this purpose, an attachment device 32, such asa bolt/washer combination or the like, is inserted through an opening 20b and mated with the IZT casing 11 to trap the return arm 20 between theattachment device 32 and the IZT casing 11. The attachment device 32cooperates with the IZT casing 11 to frictionally prevent the return arm20 from moving. At this time, under the influence of the biasing means36, projection 22 a of the control arm 22 and projection 20 a of thereturn arm 20 will be in alignment. This alignment of the projections 22a and 20 a establishes the neutral position. In the neutral position,both the inner scissor return arm 24 and the outer scissor return arm 26are in contact with the projection 22 a of the control arm 22 asparticularly illustrated in FIG. 5. Specifically, arm 24 a of the innerscissor return arm 24 and arm 26 a of the outer scissor return arm 26will be brought into contact with the projection 22 a of the control arm22 under the influence of the biasing means 36.

When the speed adjusting mechanism 16 is rotated under the influence ofa driving link to drive the axle shaft 12 in the reverse direction, theprojection 22 a of the control arm 22 will contact the arm 24 a of theinner scissor return arm 24. As a result of this contact, movement ofthe control arm 22 will also result in the movement of the inner scissorreturn arm 24. Meanwhile, the arm 26 a of the outer scissor return arm26 is prevented from moving as it remains in contact with the projection20 a of the return arm 20 as is illustrated in FIG. 6.

Once the influence of the driving link is removed from the control arm22, the biasing means 36 will cause the inner scissor return arm 24 tomove toward the outer scissor return arm 26 that is prevented frommoving through its contact with the projection 20 a of the return arm20. During this movement of the inner scissor return arm 24, the innerscissor return arm 24 will contact the control arm 22 to also move thecontrol arm 22 towards the stationary outer scissor return arm 26 andthe projection 20 a of the return arm 20. The movement of the innerscissor return arm 24 and the control arm 22 caused by the biasing means36 will continue until the projection 22 a of the control arm 22 alignswith the projection 20 a of the return arm 20 and both the inner andouter scissor return arms 24/26 contact the projection 22 a of thecontrol arm 22. Thus, under the influence of the biasing means 36 thetrunnion arm 14 is returned to the neutral position illustrated in FIG.5.

When the speed adjusting mechanism 16 is rotated under the influence ofa driving link to drive the axle shaft 12 in the forward direction, theprojection 22 a of the control arm 22 will contact the arm 26 a of theouter scissor return arm 26. As a result of this contact, movement ofthe control arm 22 will also result in the movement of the outer scissorreturn arm 26. Meanwhile, the arm 24 a of the inner scissor return arm24 is prevented from moving as it remains in contact with the projection20 a of the return arm 20 as is illustrated in FIG. 7.

Once the influence of the driving link is removed from the control arm22, the biasing means 36 will cause the outer scissor return arm 26 tomove towards the inner scissor return arm 24 that is in contact with theprojection 20 a of the return arm 20. During this movement of the outerscissor return arm 26, the outer scissor return arm 26 will contact thecontrol arm 22 to also move the control arm 22 towards the inner scissorreturn arm 24 and the projection 20 a of the return arm 20. The movementof the outer scissor return arm 26 and the control arm 22 caused by thebiasing means 36 will continue until the projection 22 a of the controlarm 22 aligns with the projection 20 a of the return arm 20 and both theinner and outer scissor return arms 24/26 contact the projection 22 a ofthe control arm 22. Thus, under the influence of the biasing means 36the trunnion arm 14 is returned to the neutral position illustrated inFIG. 5.

To prevent wear of the components that comprise the speed adjustingmechanism 16, the inner and outer scissor return arms 24/26 can beconstructed with a hardened wear surface. By way of example, the innerand outer scissor return arms 24/26 can be provided with a ZincDiChromate, “Nitrotec” or other corrosion and wear resistant finish. Forthis same purpose optional wear resistant washers 40/42 can bepositioned between the control arm 22 and inner scissor return arm 24and the inner scissor return arm 24 and the outer scissor return arm 26,respectively. Such washers can be constructed of a nylon material.Additionally, a spacer 44 can be mounted over the end of the trunnionarm 14 about which the scissor return arms 24/26 may rotate.

For allowing the speed control mechanism 16 to be placed in a pluralityof different orientations, as illustrated in FIGS. 8a-8 h, the returnarm 20 is provided with a plurality of openings 20 b through which theattachment device 32 may pass. Preferably the openings are spaced atapproximately 180 degree or 90 degree intervals. In this manner, thespeed control mechanism 16 has the advantage of allowing the neutralmarking position to be placed at a plurality of different positions withrespect to the IZT 10 casing. This is particularly useful in the case ofthe subject IZT which can be configured for both left-handed andright-handed drive in that it allows a single control mechanism to bemanufactured and used without regard to the ultimate configuration ofthe IZT and without regard to the positioning of the links used to drivethe control arm 22.

For this same purpose, the control arm 22 can also be configured toallow it to be mounted on the trunnion arm 24 in a plurality ofdifferent positions. Specifically, if the control arm 22 is adapted tocooperate with the attachment device 32 to limit the degree of movementof the control arm 22, multiple cooperating elements can be provided tothe control arm 22. By way of example, the control arm 22 can beprovided with an opening 22 b in which is disposed the attachment device32. In this manner, when the edges of the opening 22 b contact theattachment device 32, the control arm 22 is prevented from being movedfurther by the driving links. Accordingly, to allow the control arm 22to be mounted on the trunnion arm 24 in a plurality of differentpositions, the control arm 22 can be provided with a plurality of spacedopenings 22 b. The openings 22 b are preferably spaced at 180 degree or90 degree intervals. Again, this is seen to particularly useful in thecase of the subject IZT which can be configured for both left-handed andright-handed drive in that it allows a single control mechanism to bemanufactured and used without regard to the ultimate configuration ofthe IZT and without regard to the positioning of the links used to drivethe control arm 22.

An alternative embodiment of this invention is depicted in FIGS. 9-12 asspeed adjusting mechanism 116, where like numerals indicate identicalstructure to that described above. This embodiment enables the user toprovide the return to neutral feature in one direction only, referred toas a unidirectional return to neutral. Specifically, arm 124 a of innerscissor return arm 124 is shortened so that it does not contactprojection 20 a of return arm 20. The effect of this arrangement can beseen most clearly in FIGS. 11 and 12; in FIG. 11, where the control arm22 is stroked in the reverse direction, arm 26 a contacts projection 20a as described above. However, in FIG. 12, where control arm 22 isstroked in the forward direction, inner scissor arm 124 a does notcontact projection 20 a and both inner scissor return arm 124 and outerscissor return arm 26 rotate together, so that there is no return forcesupplied by biasing means 36.

Another embodiment of the unidirectional return to neutral feature isshown in FIGS. 13-16, where like numerals indicate identical structure.This alternative embodiment comprises return arm 220 having projection220 a; control arm 222 having projection 222 a; inner scissor return arm224 having arm 224 a; and outer scissor return arm 226 having arm 226 a.A biasing means 36 is connected to arms 226 b and 224 b, of outerscissor return arm 226 and inner scissor return arm 224, respectively.As shown most clearly in FIGS. 14 and 16, projection 222 a is shaped topass behind and not engage arm 226 a. Thus, in the forward direction asshown in FIG. 16, projection 222 a passes behind arm 226 a and no returnforce is applied. When the unit is stroked in reverse, however, as shownin FIG. 15, projection 222 a engages arm 224 a and a return force iscreated in a manner similar to that described above. Thus, in both ofthese alternative embodiments in FIGS. 9-16, a unidirectional return toneutral is provided. While both embodiments show the return to neutralmechanism engaged in the reverse direction and not engaged in theforward direction, this is for convenience only. It will be understoodthat the orientation could easily be reversed within the scope of thisinvention, so that the return to neutral force is provided in theforward direction but not the reverse direction.

While specific embodiments of the invention have been described indetail, it will be appreciated by those skilled in the art that variousmodifications and alternatives to those details could be developed inlight of the overall teachings of the disclosure. Accordingly, theparticular arrangement disclosed is meant to be illustrative only andnot limiting as to the scope of the invention which is to be given thefull breadth of the appended claims and any equivalents thereof.

What is claimed is:
 1. A hydraulic drive apparatus, comprising a casinghaving a hydraulic pump mounted therein; a moveable swash platecooperable with the hydraulic pump for controlling the output thereof; arotatable shaft coupled to the moveable swash plate and extending fromthe casing; and a speed adjusting mechanism mounted to the casing anddrivingly coupled to the rotatable shaft, the speed adjusting mechanismcomprising: a control arm engaged to the rotatable shaft to move theswash plate between forward and reverse positions; and a unidirectionalreturn to neutral mechanism for biasing and returning the rotatableshaft to a neutral position when the shaft is in one of the reverse orforward positions but not from the other position.
 2. A transaxle havinga casing and comprising: a hydraulic transmission mounted within thecasing and comprising a rotatable hydraulic pump and a moveable swashplate cooperable with the rotatable hydraulic pump for controlling theoutput thereof; a rotatable trunnion arm coupled to the moveable swashplate and extending from the casing; and a speed adjusting mechanismmounted to the casing and drivingly coupled to the rotatable trunnionarm, the speed adjusting mechanism being used to rotate the rotatabletrunnion arm to change the orientation of the swash plate, the speedadjusting mechanism including a unidirectional return to neutralmechanism for biasing and moving the rotatable trunnion arm from one ofa forward driving or reverse driving position to a neutral position. 3.The transaxle as set forth in claim 2, wherein the speed adjustingmechanism is further comprised of a return arm that is mounted to thecasing in a fixed position indicative of the neutral position, a controlarm that is mounted to the rotatable trunnion arm, and a pair of scissorreturn arms only one of which is adapted to move the control arm incooperation with the return arm for the purpose of moving the rotatabletrunnion arm to the neutral position.
 4. The transaxle as set forth inclaim 3, wherein the return arm has a plurality of openings foraccepting a bolt used to mount the return arm to the casing and thecontrol arm has a plurality of openings adapted to engage the bolt tolimit the degree of control arm movement.
 5. The transaxle as recited inclaim 3, further comprising a spring linking the pair of scissor returnarms that places a bias on the scissor return arms that acts to move thecontrol arm and the rotatable trunnion arm toward the neutral position.6. The transaxle as set forth in claim 2, where the transaxle is a zeroturn transaxle.
 7. The transaxle as recited in claim 4, wherein theplurality of openings in the return arm are spaced by approximately 180degrees.
 8. The transaxle as recited in claim 4, wherein the pluralityof openings in the return arm are spaced by approximately 90 degrees. 9.The transaxle as recited in claim 4, wherein the plurality of openingsin the control arm are spaced by approximately 180 degrees.
 10. A speedadjusting mechanism for use in connection with a hydrostatic driveapparatus comprising a casing in which is mounted a rotating hydraulicpump, a moveable swash plate cooperable with the rotating pump forcontrolling the output thereof, and a trunnion arm linked to themoveable swash plate for adjusting the orientation of the swash plate,the speed adjusting mechanism comprising: a return arm having aplurality of openings for accepting a bolt to mount the return arm inone of a plurality of fixed positions indicative of a neutral positionof the trunnion arm in which the swash plate does not influence theoutput of the hydraulic pump; a control arm adapted to be mounted to andmove the trunnion arm; and a pair of scissor return arms only one ofwhich is adapted to move the control arm in cooperation with the returnarm for the purpose of moving the trunnion arm to the neutral position;wherein the control arm is adapted to be mounted to the trunnion arm inone of a plurality of different positions and the mounted position ofthe return arm corresponds to the mounted position of the control armsuch that the return arm is aligned with the control arm to establishthe neutral position.
 11. A speed adjusting mechanism for use inconnection with a hydrostatic transaxle comprising a casing in which ismounted a rotating hydraulic pump in fluid communication with a rotatinghydraulic motor, a moveable swash plate cooperable with the rotatingpump for controlling the speed and direction of rotation of thehydraulic motor, and a trunnion arm linked to the moveable swash platefor adjusting the orientation of the swash plate, the speed adjustingmechanism comprising: a return arm having a plurality of openings foraccepting an attachment device to mount the return arm to the casing; acontrol arm adapted to be mounted to and move the trunnion arm; and apair of scissor return arms only one of which is adapted to move thecontrol arm in cooperation with the return arm for the purpose of movingthe trunnion arm to a neutral position established by the position ofthe return arm, the neutral position corresponding to a trunnion armposition in which the swash plate does not influence the speed anddirection of rotation of the hydraulic motor; wherein the control armhas a plurality of openings adapted to engage the attachment device tolimit the degree of control arm movement.
 12. The speed adjustingmechanism as recited in claim 11, wherein the plurality of openings inthe return arm are spaced by approximately 90 degrees.
 13. The speedadjusting mechanism as recited in claim 11, wherein the plurality ofopenings in the control arm are spaced by approximately 180 degrees. 14.The speed adjusting mechanism as recited in claim 11, wherein theplurality of openings in the control arm are spaced by approximately 180degrees.
 15. An axle driving apparatus, comprising: a casing having anopening for accepting an attachment device; a hydraulic transmissionmounted within the casing comprising a rotatable hydraulic pump in fluidcommunication with a rotatable hydraulic motor, a moveable swash platecooperable with the rotatable hydraulic pump for controlling the speedand direction of rotation of the rotatable hydraulic motor, and arotatable trunnion arm coupled to the swash plate having a trunnion armextending from the casing; an axle shaft carried by the casing anddriven by the hydraulic motor; a speed adjusting mechanism mounted tothe casing and drivingly coupled to the rotatable trunnion arm to rotatethe trunnion arm to change the orientation of the swash plate, the speedadjusting mechanism having a return to neutral mechanism comprising areturn arm having a plurality of openings for accepting the attachmentdevice which is used to mount the return arm to the casing in one of aplurality of fixed positions indicative of a neutral position of thetruing trunnion arm in which swash plate does not influence the speedand direction of the rotation of the hydraulic motor, a control armmounted to the trunnion arm, and a pair of scissor return arms only oneof which is adapted to move the control arm in cooperation with thereturn arm for the purpose of moving the trunnion arm to the neutralposition; wherein the control arm is adapted to be mounted to thetrunnion arm in one of a plurality of different positions and themounted position of the return arm corresponds to the mounted positionof the control arm such that the return arm is aligned with the controlarm to establish the neutral position.
 16. A zero-turn transaxle,comprising: a casing adapted to carry a single axle in one of aleft-handed or a right-handed drive position; a hydraulic transmissionmounted within the casing comprising a rotatable hydraulic pump in fluidcommunication with a rotatable hydraulic motor and a moveable swashplate cooperable with the rotatable hydraulic pump for controlling thespeed and direction of rotation of the rotatable hydraulic motor, therotatable hydraulic motor being used to drive the axle; a rotatabletrunnion arm coupled to the moveable swash plate, the rotatable trunnionarm extending from the casing; and a speed adjusting mechanism mountedto the casing and drivingly coupled to the rotatable trunnion arm, thespeed adjusting mechanism being used to rotate the rotatable trunnionarm to change the orientation of the swash plate to change the speed anddirection of rotation of the rotatable hydraulic motor; wherein thespeed adjusting mechanism further comprises a unidirectional return toneutral mechanism mounted to the rotatable trunnion arm in one of aplurality of rotational orientations with respect to the rotatabletrunnion arm, the speed adjusting mechanism being adapted to bias andmove the rotatable trunnion arm from one of a forward or reverse drivingposition to a neutral position where the swash plate does not influencethe speed and direction of rotation of the rotatable hydraulic motor.17. The zero-turn transaxle as recited in claim 16, wherein the returnto neutral mechanism comprises a return arm that is mounted to thecasing, a control arm that is mounted to the rotatable trunnion arm, anda pair of scissor return arms only one of which is adapted to move thecontrol arm in cooperation with the return arm for the purpose of movingthe rotatable trunnion arm to the neutral position.
 18. The zero-turntransaxle as recited in claim 17, further comprising a spring linkingthe pair of scissor return arms that places a bias on the scissor returnarms that acts to move the control arm and the rotatable trunnion armtoward the neutral position.
 19. The zero-turn transaxle as recited inclaim 17, further comprising a bolt adapted to mate with the casing andwherein the return arm has a plurality of openings each adapted toaccept the bolt, the plurality of openings provided for use inestablishing the rotational orientation of the speed adjusting mechanismwith respect to the rotatable trunnion arm.
 20. The zero-turn transaxleas recited in claim 19, wherein the control arm has a plurality ofopenings adapted to engage the bolt to provide a limit to the degree ofmovement afforded to the control arm.
 21. A speed adjusting mechanismfor use in connection with a hydraulic pump carried in a casing and atrunnion arm linked to a swash plate to move the swash plate to affectfluid displacement of the hydraulic pump, the speed adjusting mechanismcomprising: a return arm adapted to be mounted to the casing in aposition indicative of a neutral position of the trunnion arm in whichthe swash plate does not influence the speed and direction of rotationof the hydraulic motor; a control arm adapted to move the trunnion arm,the control arm being mated with the trunnion arm such that the returnarm and the control arm are alignable to establish the neutral positionof the trunnion arm; and a pair of scissor return arms wherein only oneof the pair of scissor return arms is adapted to move the control arm incooperation with the return arm for the purpose of moving the trunnionarm to the neutral position.
 22. The speed adjusting mechanism asrecited in claim 21, wherein the return arm has a plurality of openingsthrough which an attachment device is adapted to pass to mount thereturn arm to the casing.
 23. The speed adjusting mechanism as recitedin claim 22, wherein the plurality of openings are spaced byapproximately 90 degrees.
 24. The speed adjusting mechanism as recitedin claim 22, wherein the plurality of openings are spaced byapproximately 180 degrees.
 25. The speed adjusting mechanism as recitedin claim 22, wherein the attachment device comprises a bolt adapted tomate with the casing.
 26. The speed adjusting mechanism as recited inclaim 22, wherein the control arm has a plurality of openings adapted toengage the bolt to limit the degree of control arm movement.
 27. Thespeed adjusting mechanism as recited in claim 26, wherein the pluralityof openings in the control arm are spaced by approximately 90 degrees.28. The speed adjusting mechanism as recited in claim 26, wherein theplurality of openings in the control arm are spaced by approximately 180degrees.
 29. The speed adjusting mechanism as recited in claim 25,wherein a first one of the pair of scissor return arms is shorter than asecond one of the pair of scissor return arms so as to allow the firstone of the pair of scissor return arms to avoid cooperation with thereturn arm for moving the control arm.
 30. The speed adjusting mechanismas recited in claim 21, wherein the control arm comprises a projectionthat is sized and arranged so as to allow engagement with only one ofthe pair of scissor return arms so as to avoid cooperation between thereturn arm and the other of the scissor return arms for moving thecontrol arm.